Output buffer and method of driving

ABSTRACT

A method is for driving an output buffer for outputting a datum of a certain voltage level with a certain slew-rate as a function of an input datum and a first enabling signal. The first enabling signal commands the buffer to a normal functioning state or to a high impedance state. The output buffer has an output stage controlled at least by a pull-up driving circuit and a pull-down driving circuit, and an enabling circuit input with the input datum and a second enabling signal and generating control signals. The control signals may be in phase or in phase opposition depending on whether the second enabling signal is active or disabled, and they are input into the respective driving circuits.

FIELD OF THE INVENTION

[0001] The present invention relates to electronic circuits, and, more particularly, to output buffers such as integrated output buffers with slew-rate regulation.

BACKGROUND OF THE INVENTION

[0002] Output buffers are used in many devices for producing an output signal with a desired slew-rate at a certain time and with a certain output voltage-current characteristic. They essentially include a pull-up device and a pull-down device driven by a circuit, and they output a datum with a certain slew-rate and voltage level characteristic when an externally generated enabling signal is active.

[0003] A basic diagram of an output buffer is shown in FIG. 3, where an input datum DATO is output (DQ_PAD) with a desired voltage and slew-rate if an enabling signal EN is active. The control circuit (FIG. 4) substantially includes a logic circuit that turns on or forces the buffer into a high impedance state by producing two pull-up and pull down control signals P and N for the pull-up and pull down driving circuits, respectively.

[0004] A simple embodiment of the tristate circuit for placing the buffer in a high impedance state is illustrated in FIG. 5. The circuit produces a pair of control signals P and N both equal to the inverted logic signal of the input signal DATO if the enabling signal EN is active or different from each other. This is done independently of the value of the input signal DATO.

[0005] Such circuits, the basic diagram of which is shown in FIG. 6, produce driving signals PGATE and NGATE that switch with a certain variable speed to regulate the slew-rate of the final stage as desired. The slew-rate of the output DQ_PAD depends in general on the load and on the charge/discharge current, which depends upon the size of the transistor of the final stage and from the gate-source voltage applied to the transistors thereof. The slew-rate may be regulated by regulating the gate-source voltage. This is because the load is generally constant and the sizes of the transistors of the final stage are established to have a desired output voltage-current characteristic.

[0006] For better comprehension of the technical problem being addressed, a case in which the output of the final stage switches from high to low will now be considered. On the pull-up transistors a voltage PGATE is applied that rapidly reaches the supply value, as illustrated in FIG. 7, and remains at this value indefinitely while the voltage NGATE increases with a certain slope.

[0007] The pull-up transistor is immediately turned off while the pull-down transistor turns on slowly. This is so that the output voltage remains at a high value minus a small voltage drop due to the below-threshold current for as long as the pull-down does not leave the below-threshold region.

[0008] After a time interval TON (turn-on time) has elapsed, the voltage NGATE overcomes the threshold voltage VTN and the output voltage drops more rapidly. In this phase the output capacitance discharges through a pull-down transistor, as illustrated in FIG. 8, but in a time that depends on the current I flowing therethrough. For this reason, the faster the switching of the signal NGATE is, the faster the current I increases. Thus, the greater the slew-rate of the output DQ_PAD will be and the shorter the response time T_val. With such prior art buffers it is not possible to produce an output datum with a relatively small slew-rate in relatively short response times.

[0009] Generally, the buffer should be able to function in different modes depending on the environment in which it works, both in terms of the voltage-current characteristic (e.g., the allowed functioning zones of the voltage-current characteristic for both transitions are shown in FIG. 1, for the case of a PCI 3.3V environment) as well as in terms of output slew-rate (e.g., in a PCI environment the slew-rate must be smaller than that normally required in a TTL environment).

[0010] Typical values of the slew-rate for a PCI environment under standard load conditions and defined by the specifications are indicated in the following table: TABLE 1 Parameter Condition Min ÷ Max Slew-rate of 0.2 Vcc ÷ 0.6 Vcc 1 ÷ 4 V/ns the raising load edge Slew-rate of 0.2 Vcc ÷ 0.6 Vcc 1 ÷ 4 V/ns the falling load edge

[0011] To make the slew-rate of the output compatible with a PCI or TTL environment, the buffer may be designed in a way such that the slew-rate is always equal to the minimum that is ever required. Unfortunately, despite its simplicity, this approach is not convenient because it unduly increases the response time T_(—) VAL of the buffer.

[0012] Referring to FIG. 2, the response time T_(—) VAL is the time that elapses from the edge of the clock signal CLK, which determines the instant in which the input datum DATO is read, to the edge of the output DQ_PAD. The response time of prior art buffers is generally the sum of the delay introduced by the logic circuits that form the buffer, of the turn-on time TON of the pull-up or pull-down transistor, and of the time required by the output DQ_PAD for switching from one value to another value with a certain slew-rate.

[0013] Because the delay caused by the logic circuits is constant, with the circuit of FIG. 3 the build-up time T_(—) VAL may be reduced only by increasing the speed of variation of the signal that drives the transistor to be turned on. Consequently, a transition of the output DQ_PAD with a slew-rate that may be excessively large for a PCI environment must be determined.

[0014] There is a need for an output buffer capable of producing an output datum with a relatively small slew-rate while ensuring an acceptable fast response. Moreover, it would be highly desirable to have an output buffer producing a datum in a certain mode of operation with a slew-rate that may be adjusted at least between two different values, though with response times practically equal or even shorter than the response times of comparable known buffers.

SUMMARY OF THE INVENTION

[0015] It is an object of the invention to provide a method of driving an output buffer that provides shorter response times than those of prior art buffers with comparable slew-rates.

[0016] Another object of the invention is a to provide a method of driving an output buffer that improves response time.

[0017] A further object of the invention is to provide an output buffer capable of outputting a datum with a reduced slew-rate and without penalizing the response time.

[0018] In contrast to prior art buffers, according to the present invention the slew-rate is controlled by regulating the turn-off and not the turn-on of the pull-up or the pull-down transistors. Considering, for example, a transition from the high state to the low state of the output DQ_PAD, instead of immediately turning off the pull-up transistor and slowly turning on the pull-down transistor, the opposite is done. That is, the pull-down transistor is turned on quickly and the pull-up transistor is turned off slowly.

[0019] This approach, despite the increase of power consumption due to the temporary short-circuiting of the supply in the output stage, reduces the time of response for the same slew-rate. This is because T_(—) VAL is no longer limited by the turn-on time TON. The increment of power absorption, due to the short-circuiting of the supply, remains within an acceptable range.

[0020] Moreover, it has been observed that the slew-rate of the output switchings may be less sensitive to temperature variations than in prior art buffers because both transistors are turned on during the output switchings. Indeed, temperature variations influence in an opposite manner the pull-up and the pull-down transistors. This is because the output is switched when both transistors are in a conduction state. Thus, the slew-rate becomes less dependent upon temperature.

[0021] By having a buffer that may have a faster response than known buffers with the same slew-rate, or a reduced slew-rate with the same response time, it is possible to implement, with few modifications, output buffers with different modes of operation. For example, buffers with a slew-rate that may be set at at least two different values, and a response time equal to or faster than that of comparable prior art buffers of similar fabrication technology may be implemented.

[0022] According to one embodiment of the invention, an IC selection signal of the functioning mode, which is applied to the tristate circuit, allows for the selection between at least two different modes of operation of the driving circuits of the pull-up and the pull-down transistors. This determines the switching of the buffer output with one or another slew-rate.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The various aspects and advantages of the invention will become even more apparent through a description of several embodiment of the invention, with reference to the attached drawings, wherein:

[0024]FIG. 1 is a graph illustrating the valid zone of operation of the output voltage-current characteristic in a PCI 3.3V environment according to the prior art;

[0025]FIG. 2 is a timing diagram of the succession of an edge of the clock signal and the output edge according to the prior art;

[0026]FIG. 3 is a schematic block diagram of an output buffer according to the prior art;

[0027]FIG. 4 is a schematic block diagram of the control circuit of the output buffer of FIG. 3;

[0028]FIG. 5 is schematic diagram of a possible hardware embodiment of the tristate circuit of FIG. 4;

[0029]FIG. 6 is a schematic diagram of prior art embodiments for driving circuits for the pull-up and the pull-down transistors;

[0030]FIG. 7 is a diagram of the driving and of the output signals of a buffer made according to the diagram of FIG. 3;

[0031]FIG. 8 is a schematic diagram of the output stage of a prior art buffer illustratively shown during switching from high to low;

[0032]FIG. 9 is a schematic block diagram of the buffer according to the present invention;

[0033]FIG. 10 is a schematic block diagram of an embodiment of the control circuit of the buffer of the invention;

[0034]FIG. 11 is schematic diagram of an embodiment of the pull-up and the pull-down logic circuits according to the invention;

[0035]FIG. 12 is a schematic diagram of the slew-rate control circuit of FIG. 10;

[0036]FIG. 13 is a schematic block diagram of the pull-up and the pull-down driving circuits of the invention;

[0037]FIG. 14 is a schematic diagram of the circuit blocks DRIVER_P and DRIVER_N of FIG. 13;

[0038]FIG. 15 is a diagram of the driving signals of the output buffer of the invention during a transition from high to low;

[0039]FIG. 16 is a schematic diagram of the output stage of the buffer of the invention during a transition from high to low;

[0040]FIG. 17 is a diagram comparing output transition characteristics of the buffer of the invention and of a prior art buffer with the same slew-rate;

[0041]FIG. 18 is a diagram comparing output transition characteristics of the buffer of the invention and of a prior art buffer with the same response time; and

[0042]FIG. 19 is a schematic block diagram of an embodiment of the output stage of the buffer of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0043] The method of the invention includes regulating the slew-rate during a high-low (low-high) transition by controlling the turning-off of the pull-up (pull-down) transistor, instead of the turning-on of the pull-down (pull-up) transistor. In this way, the build-up time T_(—) VAL is reduced in a non-negligible way because it is no longer necessary to wait the turn-on time of the pull-up or the pull-down transistor to switch the output.

[0044] When the buffer switches from a tristate condition to normal functioning, it is necessary to drive the turning-on of the pull-up as well as of the pull-down transistors, as it happens in common output buffers. However, in this case the response time T_(—) VAL is relatively short, even though it includes the turn-on time, because it does not include the signal propagation delays of the logic circuits of the buffer. In fact, in this case, the logic circuits have already produced the input datum before activating the output buffer enabling signal.

[0045] As already noted, another feature of the buffer of the invention is that it may function in at least two different modes. That is, outputting an output datum with a relatively reduced slew-rate to meet the requirement of PCI and TTL environments, without penalizing the response time T_(—) VAL.

[0046] It is not sufficient to drive the turning-off of the pull-up and pull-down transistors for switching the output with a desired slew-rate. In fact, the transistors of the output stage are in an off state when switching from tristate to normal functioning. A circuit that regulates the turning on of the transistors of the output stage with a pre-established dynamic, according to the invention, is shown in FIG. 9.

[0047] As with the prior output buffers, the buffer of the invention may also have a tristate circuit receiving the input datum DATO and a disabling signal EN_(—) DEL. It may further have a pair of pull-up and pull-down driving circuits for the output stage. One advantageous feature of the circuit of FIG. 9 is that it includes a slew-rate control circuit for the first switching from a tristate condition that produces regulation signals for turning on the pull-up (ON_P) and the pull-down (ON_N).

[0048] These signals are input into the respective driving circuits as a function of the datum to be output DATO and of an enabling signal EN. The signal EN_(—) DEL is a delayed replica of the signal EN, thus the regulation signals ON_P and ON_N, when resuming normal operation from tristate, are output ahead of the control signals P and N. In this way the drive signals PGATE and NGATE will be generated with the desired dynamic before the new control signal is received.

[0049] The driving circuits of the pull-up and of the pull-down transistors are essentially NOR and NAND logic gates, respectively, input with the inverted logic signal of the respective control signal (P, N) and with the respective regulation signal (ON_P, ON_N). According to one aspect of this invention, the nodes of these gates on which the respective regulation signals are present drive transistors that are more resistive than transistors driven by the other nodes.

[0050] When resuming operation from tristate, the respective regulation signal varies, while the control signals P and N are still at the values assumed during the tristate phase because EN_(—) DEL is still disabled. Therefore, PGATE and NGATE correspond to the inverted logic signals of ON_P and ON_N, respectively. The speed of the variation (i.e. the dynamic) of the driving signals (PGATE, NGATE) may be established by sizing the transistors driven by the input of the logic gates on which the regulation signal is applied to be more or less resistive.

[0051] The pull-up and pull-down driving circuits for the transistors of the output stage of the buffer may be implemented as shown in FIG. 14, in a manner that will be understood by those of skill in the art. For better illustrating the functioning of the buffer of the invention, the case in which a transition from high to low of the output DQ_PAD of the buffer must be produced will not be considered.

[0052] As illustrated in FIG. 15, the signal NGATE switches quickly to its active value, turning on the pull-down transistor, while the signal PGATE raises relatively more slowly. Thus, the current circulating in the pull-up transistor is slowly reduced and eventually turns off. The output capacitance discharges with a current I that is the difference between the current I_(p) provided by the pull-up transistor and the current I_(n) drained by the pull-down transistor, as illustratively shown in FIG. 16. The speed of variation of the current I_(p) determines the slew-rate of the output.

[0053] In practice, the buffer of the invention has a response time T_(—) VAL _(—) NEW noticeably smaller than the response time T_(—) VAL _(—) OLD of a comparable prior art buffer with the same slew-rate, or vice-versa, as shown in FIGS. 17 and 18. This is because it is no longer necessary to wait for the turn-on time TON of the transistors of the output stage.

[0054] Because the slew-rate of the buffer of the invention may be reduced without significantly penalizing the response time, it is possible to realize an output buffer whose functioning characteristics are selectable between a first mode with a relatively high slew-rate and short response time that may be suitable in a TTL environment, and a second mode with a relatively low slew-rate and longer response time that may be most suitable in a PCI environment. To this end, a selection signal IC may be input into the buffer to make the control circuit of the slew-rate produce regulation signals ON_P and ON_N according to the selected mode.

[0055] By way of a example, the particular case in which the buffer must function in either PCI or TTL logic based upon whether the signal IC is active or disabled will now be considered. A simple embodiment of the slew-rate control circuit is illustrated in FIG. 12, as will be understood by those of skill in the art. It is worth noting that if the signal IC is not active, i.e., the PCI functioning mode is not selected, the control circuit of the slew-rate is practically disabled because it is convenient to have a large slew-rate with a short response time.

[0056] According to one embodiment of the invention, the output stage may be dimensionally split, as shown in FIG. 19, by using a pair of pull-up transistors and a pair of pull-down transistors (FINALE 1, FINALE 2). In this way, the output stage has effective sizes of the transistors that, when using both transistors of each pair, satisfy the specifications of a PCI environment, yet while using only one transistor of each pair the sizes satisfy the more relaxed specifications of a TTL environment.

[0057] For driving the output stage, the necessary control signals are generated, for example, by using the circuit of FIG. 10. It will be observed that the tristate circuit produces the signals P and N for the respective logic circuits, indicated in FIG. 10 as the pull-up and pull-down selection circuits that produce first and second pull-up and pull-down control signals P1, N1 and P2, N2, respectively.

[0058] The first control signals P1 and N1 are used for controlling the functioning of the pull-up and pull-down transistor pair that must function in PCI as well as TTL environments. The second control signals P2 and N2 are used to control the functioning of the pull-up and pull-down transistor pair that must remain inactive in a TTL environment. These logic circuits, one embodiment of which is depicted in FIG. 11, produce the pair of signals P2 and N2 by ORing the signal P and the inverted logic signal of the selection input IC and by ANDing N and IC, respectively. The pair of signals P1 and N1 are replicas of the respective control signals P and N suitably delayed by the two-inverter chain to be output simultaneously with the other pair of signals P2 and N2.

[0059] The pull-up and pull-down driving circuits are also doubled, as illustrated in FIG. 13, to separately control the functioning of the two pairs of pull-up and pull-down transistors of the output stage. Of course, it is also possible for both output transistor pairs to work irrespectively of the logic environment. Yet, in this case the output stage would be over-dimensioned when functioning in TTL mode, with consequent drawbacks of greater noise and power dissipation.

[0060] The buffer of the invention switches its output with a slew-rate that is less dependent upon temperature and supply voltage than prior art buffers, as may be readily observed by comparing the data in Table 2, below: TABLE 2 Working Slew-rate Slew-rate T_VAL T_VAL Conditions (prior art) (invention) (prior art) (invention)   3 V; 85° C. 1.75 V/ns 1.4 V/ns 14.4 ns 8.2 ns 3.3 V; 27° C. 3.0 V/ns 2.0 V/ns 10.5 ns 6.5 ns 3.6 V; 0° C. 4.1 V/ns 2.7 V/ns  8.3 ns 5.4 ns

[0061] These results may be explained by considering that temperature and voltage variations induce on the pull-up and the pull-down opposite effects that, to some extent, balance one another. This is because in the buffer of the invention both the pull-up and the pull-down are in conduction state when switching the output. The fact that there is a time interval in which both transistors are conducting, thus short-circuiting the supply, causes a momentary power dissipation that remains well within an acceptable range. 

1. An output buffer for outputting a datum (DQ _(—) PAD) of a certain voltage level with a certain slew-rate, in function of an input datum (DATO) and of a first enabling signal (EN) that commands the buffer to a normal functioning state or to a high impedance state, having an output stage constituted at least by a pull-up transistor and a pull-down transistor, controlled at least by a respective pull-up driving circuit (DRIVER_P) and a pull-down driving circuit (DRIVER_N), an enabling circuit (TRISTATE) input with said input datum (DATO) and a second enabling signal (EN_(—) DEL) and generating control signals (P, N), in phase or in phase opposition depending on whether said second enabling signal (EN_(—) DEL) is active or disabled, that are input to the respective driving circuits (DRIVER_P, DRIVER_N), characterized in that said second enabling signal (EN_(—) DEL) is a delayed replica of said enabling signal (EN); said buffer comprises a regulation circuit of the slew-rate producing regulation signals of the slew-rate of the pull-up (ON_P) and of the pull-down (ON_N) in function of said first enabling signal (EN) and of said input datum (DATO); the pull-up driving circuit (DRIVER_P) comprises at least a logic NOR gate receiving on a first input node the inverted logic signal of the respective control signal (P) and on a second input node, driving transistors that are more resistive than the transistors driven by the first input node, the respective slew-rate regulation signal (ON_P); the pull-down driving circuit (DRIVER_N) comprises at least a logic NAND gate receiving on a first input node the inverted logic signal of the respective control signal (N) and on a second input node, driving transistors that are more resistive than the transistors driven by the first input node, the respective slew-rate regulation signal (ON_N).
 2. The buffer of claim 1, wherein said slew-rate regulation signal (ON_P) of the pull-up is produced by ANDing said first enabling signal (EN) and said input datum (DATO), while said slew-rate regulation signal (ON_N) of the pull-down is produced by NANDing said first enabling signal (EN) and the inverted logic signal of the input datum (DATO).
 3. The buffer of claim 1, wherein a mode selection signal (IC), selecting one or the other of two different values of slew-rate, is applied to said slew-rate regulation circuit ANDing said enabling input (EN), the input datum (DATO) and said mode selection signal (IC), for producing the regulation signal (ON_P) for the pull-up, and NANDing the enabling input (EN), the inverted logic signal of the input datum (DATO) and said selection signal (IC) for producing the regulation signal (ON_N) for the pull-down.
 4. The buffer according to any of claims from 1 to 3, wherein said pull-up driving circuit (DRIVER_P), producing a respective driving signal (PGATE), includes a cascade of an even number of inverters and a NMOS transistor connected between the output node of the driving circuit and the reference potential, and the output inverter of said cascade being supplied through a PMOS transistor connected to the supply (VDD) that is more resistive of said NMOS transistor, both said transistors being controlled by said slew-rate regulation signal (ON_P) for the pull-up; said pull-down driving circuit (DRIVER_N), producing a respective driving signal (NGATE), includes a cascade of an even number of inverters and a PMOS transistor connected between the output node of the driving circuit and the supply node (VDD), the output inverter of said cascade being supplied through a NMOS transistor connected to the reference voltage node of the circuit more resistive than said PMOS transistor, both said transistors being controlled by said slew-rate regulation signal (ON_N) for the pull-down.
 5. The buffer according to one of claims 3 and 4, comprising a logic circuit for the pull-up producing a first control signal (P1) for the pull-up as a delayed replica of the respective control signal (P), and a second control signal (P2) for the pull-up by ORing the respective control signal (P) and the inverted logic signal of the selection signal (IC), a pair of identical pull-up driving circuits (DRIVER_P), input with the respective regulation signal (ON_P) and with control signals first (P1) and second (P2) for the pull-up, respectively, producing respective driving signals first (PGATE 1) and second (PGATE 2); a logic circuit for the pull-down producing a first control signal (N1) for the pull-down as delayed replica of the respective control signal (N), and a second control signal(N2) for the pull-down by ANDing the respective control signal (N) and the selection signal (IC), a pair of pull-down driving circuits (DRIVER_N), input with the respective regulation signal (ON_N) and with the control signals first (N1) and second (N2) for the pull-down, respectively, producing respective driving signals first (NGATE 1) and second (NGATE 2) said output stage comprising a pair of pull-up and pull-down stages first (FINALE 1) and second (FINALE 2), controlled by the driving signals first (PGATE 1, NGATE 1) and second (PGATE 1, NGATE 2), respectively, producing said output datum (DQ _(—) PAD) on an output common node.
 6. The buffer according to any of claims from 1 to 5, wherein said enabling circuit (TRISTATE) produces the control signal (P) or the pull-up by NANDing the second enabling signal (EN_(—) DEL) and the input datum (DATO), and produces the control signal (N) for the pull-down by NORing the inverted logic signal of the second enabling signal (EN_(—) DEL) and the input datum (DATO).
 7. A method for driving an output buffer for outputting a datum (DQ _(—) PAD) of a certain voltage level with a certain slew-rate, in function of an input datum (DATO) and of a first enabling signal (EN) that commands the buffer to a normal functioning state or to a high impedance state, having an output stage constituted at least by a pull-up transistor and a pull-down transistor, controlled at least by a respective pull-up driving circuit (DRIVER_P) and a pull-down driving circuit (DRIVER_N), an enabling circuit (TRISTATE) input with said input datum (DATO) and a second enabling signal (EN_(—) DEL) and generating control signals (P, N), in phase or in phase opposition depending on whether said second enabling signal (EN_(—) DEL) is active or disabled, that are input to the respective driving circuits (DRIVER_P, DRIVER_N), said method comprising the step of turning on pull-up or pull-down transistors according to a pre-established slew-rate, switching the output datum (DATO) when resuming from a high impedance state to a normal functioning state, switching the output datum (DATO) after having resumed from the high impedance state by turning on the transistor previously turned off and successively turning off the transistor previously turned on, according to a pre-established slew-rate. 